Reduction Of Absorption Coefficient Research Articles

Increase in precipitation scavenging contributes to long-term reductions of light-absorbing aerosol in the Arctic

Abstract. We investigated long-term changes using a harmonised 22-year data set of aerosol light absorption measurements, in conjunction with air mass history and aerosol source analysis. The measurements were performed at Zeppelin Observatory, Svalbard, from 2002 to 2023. We report a statistically significant decreasing long-term trend for the light absorption coefficient. However, the last 8 years of 2016–2023 showed a slight increase in the magnitude of the light absorption coefficient for the Arctic haze season. In addition, we observed an increasing trend in the single-scattering albedo from 2002 to 2023. Five distinct source regions, representing different transport pathways, were identified. The trends involving air masses from the five regions showed decreasing absorption coefficients, except for the air masses from Eurasia. We show that the changes in the occurrences of each transport pathway cannot explain the reductions in the absorption coefficient observed at the Zeppelin station. An increase in contributions of air masses from more marine regions, with lower absorption coefficients, is compensated for by an influence from high-emission regions. The proportion of air masses en route to Zeppelin, which have been influenced by active fires, has undergone a noticeable increase starting in 2015. However, this increase has not impacted the long-term trends in the concentration of light-absorbing aerosol. Along with aerosol optical properties, we also show an increasing trend in accumulated surface precipitation experienced by air masses en route to the Zeppelin Observatory. We argue that the increase in precipitation, as experienced by air masses arriving at the station, can explain a quarter of the long-term reduction in the light absorption coefficient. We emphasise that meteorological conditions en route to the Zeppelin Observatory are critical for understanding the observed trends.

Bandgap tunability endowed by isovalent sulphur doping in SeTe glassy films: Correlation with Kastner’s and single oscillator models

Thin films of sulphur (S) incorporated SeTe with chemical composition of Se 80 Te 20-x S x (x = 0, 5, 10, 15, indicate the weight percentage of sulphur incorporated) were deposited via vacuum evaporation technique. S incorporation in the SeTe alloy led to reduction in the glass transition temperature from 333 K to 314 K. The investigated films were amorphous in nature with a thickness of ∼250 nm. In comparison to pure selenium telluride (SeTe) film, the electrical conductivity exhibited an enhancement up to 10 wt % sulphur doping above which a decrease in electrical conductivity was apparent owing to microstructural porosity and enhanced bandgap. It is inferred that reduced grain size and dense morphology led to higher value of absorption coefficient as well as enhancement in the electrical conductivity. Interestingly, Te replacement with S results in monotonic increase in bandgap from 1.81 to 2.60eV and a reduction in absorption coefficient. The trend of the observed bandgap for the S incorporated samples is consistent with the Kastner and single oscillator models. • S doped SeTe alloys are deposited by vacuum evaporation technique. • Glass transition temperature reduces with S incorporation. • Bandgap increases with increasing S content. • S insertion in the alloy results in low nucleation rate and better glass forming ability. • Dense microstructure and more defects led to enhanced conductivity.

On Determination of Acid Site Densities on Sulfated Oxides

Combined infrared and gravimetric studies have been performed of pyridine adsorption on sulfated and non-sulfated silica-zirconia mixed oxides in order to quantify the site densities of both Bronsted and Lewis acid sites. The values determined for the integrated molar absorption coefficients for the non-sulfated silica-zirconia mixed oxides were 1.24 cm μmol-1 at 1540 cm-1 and 1.56 cm μmol-1 at 1450 cm-1 and were independent of the Si:Zr ratio and other pre-treatment and preparation variables. On the other hand, values ranging from 0.64 to 1.66 cm μmol-1 at 1540 cm-1 for the pyridinium ion and from 1.00 to 2.08 cm μmol-1 for the Lewis-bound pyridine were determined for the sulfated samples. It is proposed that the reduction in absorption coefficient for the pyridinium ion with increasing sulfate density corresponds with the transformation of monomeric to dimeric or polymeric sulfate species which bring two adjacent adsorbed pyridine molecules into close proximity. The findings of this study raise considerable concern over the validity of acid site densities determined for sulfated materials where calculations have been performed based on literature values for the absorption coefficients.

A new interpretation of reduction in absorption coefficient at and near Laue diffraction maximum in thin and perfect crystals on the basis of dynamical theory of X-ray diffraction

Abstract By analysing in detail the existing plane wave dynamical theory of X-ray diffraction for thin (μt sec θ ∼ 0.30) and perfect crystal we demonstrate the presence of forward diffracted beam when the crystal is oriented for Laue diffraction. Recently, these features have been demonstrated experimentally by Lal et al. (Solid State Comm. 81 (1992) 461–465 & 96 (1995) 33–36) in the case of diamond crystals. We also show that a residual direct beam is implicitly built in the expression for forward diffracted beam intensity. It has been shown that there is a substantial decrease in absorption coefficient, μ, of thin perfect crystals for exploring X-ray beam around Laue diffraction maximum. The computed fractional change in μ, Δμ/μ, at the [unk]20 diffraction peak of a thin (111) diamond crystal for MoK α1 exploring radiation is found to be in reasonably good agreement with the experimentally observed value (Lal et al. Acta Crystallogr. A52 (1996) C-493). The results reported here are of general nature and applicable to all crystals.

Temperature dependence of nonlinear absorption in InP doping superlattices

Absorptive optical nonlinearity in InP doping superlattices grown by vapor-phase epitaxy is investigated in a 2–300 K temperature range. The absorption spectra have tails below InP band-gap energy, caused by the Franz–Keldysh effect due to the internal electric field. The absorption tail is almost temperature independent. This means that the periodic electric potential in the doping superlattice, responsible for the nonlinearity, is unchanged across this temperature range. Nonlinear absorption is evaluated by the pump-probe method. The large reduction in absorption coefficient is obtained even with a weak excitation light intensity. This enhancement in nonlinearity is due to prolongation of the excited-carrier lifetime. Nonlinear absorption increases as temperature decreases. This is due to the temperature dependence of the carrier lifetime.